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lesson12 introduction to instrumentation and control in thermal power plant

lesson12 introduction to instrumentation and control in thermal power plant
lesson12 introduction to instrumentation and control in thermal power plant

Lesson12 Introduction to Instrumentation and control in thermal power plant Instrumentation and control (I&C) is an integral part of a coal-fired power station.

A modern,advanced I&C system plays a major role in the profitable operation of a plant by achieving maximum availability,reliability,flexibility,maintain ability and efficiency. These systems can also assist in maintaining emissions compliance. The I&C chain begins with sensors that detect measured values. Controllers receive these values,upon which a control strategy is activated. The response,where and when required, moves to final actuating control elements to modify the affected process. This loop repeats over and over during plant operation through a complex and multi-level communications schemes.

“Smart”field devices,including sensors and actuators continue to be developed in order to simplify and improve the control process. For instance,in the past a transmitter was used to measure pressure,temperature and flow(differential pressure) from remote locations. A signal representing the calibrated value of that measurement is sent to the control system located in or near to the central control room. The signal is then conditioned to provide a usable signal to the control system,indicator,or recorder. The smart transmitters available today can condition the signal locally-for example,square root extraction to convert differential pressure to flow and temperature compensation for flow measurements. The smart transmitter can also provide remote configuration,reranging,rezeroing,calibration and diagnostics features to provide direct access to the filed device from the control room,virtually eliminating periodic maintenance visits to the instruments.

The two main control platforms that are used in coal-fired power station are the distributed control system (DCS) and the programmable logic control(PLC). Personal computer (PC) based hardware and software has only recently been introduced in power plant control. With the fast development,increasing power and reduced cost of personal computers,PC-based control is expected to become a further plat form for future development and growth. Today new coal-fired power plants are,in general,built with modern,advanced DCS/PLC and a large number of existing coal-fired power stations have been retrofitted with advanced digital systems in many countries throughout the world.

Basic Control Theory

Boiler control means the regulation of the boiler outlet conditions of steam flow,pressure and temperature to their desired values. In control terminology,the boiler outlet steam conditions are called the outputs or controlled variables. The desired

values of the outlet conditions are the setpoints or input demand signals. The quantities of fuel,air and water are adjusted to obtain the desired outlet steam conditions and are called the manipulated or control variables.

Disruptive influencies on the boiler,both internal and external, such as fuel heat content variations,unit load change or change in cycle efficiency,are called disturbance inputs. The controller or control system looks at the desired(setpoints) and actual(output variables) values of the outlet steam conditions and adjusts the amounts of fuel. Air and water(manipulated variables) to make the outlet conditions match their desired values. The controller can be manual,with an operator making the adjustments,or automatic,with a pneumatic system,electronic analog computer,or a digital computer,making the adjustments. A block diagram of a simple boiler control system is shown in Fig.1,which represents the control of one output variable,steam pressure,in a hypothetical boiler. The setpoints for steam pressure and steam flow will be used as reference values,and the manipulated variable is the fuel valve position. Disturbance inputs act on the boiler,the system to be controlled.

Fig.1 Simplified block diagram of a boiler control function

While it is theoretically possible to operate a boiler with manual control,the operator must maintain tedious,constant watch for a disturbance. Time is needed for the boiler to respond to a correction and this can lead to overcorrection with further upset to the boiler. An automatic controller,on the other hand,once properly tuned will make the proper adjustment quickly to minimize upsets and will control the system more accurately and reliably.

Feedforward/Feedback control

In a closed loop system,the controlled variable must deviate from its setpoint before the controller initiates a corrective action. The open loop system therefore has a faster response because it takes corrective action before the controller variable starts to change. When the open loop or feedforward system is combined with the closedloop or feedback system,the result is a fast response system that can

compensate for changes in the calibration curve. Fig.2 represents a feedforward/feedback control system for the control function of Fig.1.

Fig.2 Feedforward/Feedback control

As the step change in load occurs, the feedforward signal immediately positions the fuel valve to meet the calibration requirements. If this function is exact, no error develops and the feedback loop has no work to do. If the calibration function is in error, the feedback loop readjusts the fuel valve position to eliminate any pressure error which may develop due to shifts in the calibration. However,the response will be faster and the magnitude of the system upset will be smaller because the feedforward signal moves the valve near its final steady-state position leaving a smaller range of action for the feedback loop.

NEW WORDS;

1、emission n排放

2、compliance [k?m'pla??ns] n. acting according to certain accepted standards

3、calibrate ['k?l?bre?t] vt.校正;调整calibration n.校准;

4、retrofit['retr??f?t] vt.改进;

5、Disruptive [d?s'r?pt?v] adj.破坏的;

6、upset n.扰动

NOTES

The smart transmitter can also provide remote configuration,reranging,rezeroing,calibration and diagnostics features to provide direct access to the filed

device from the control room,virtually eliminating periodic maintenance visits to the instruments.

智能变送器也可以提供远程重新组态、重新更改量程、重新调零、校正及诊断功能,从而提供一个从控制室到设备的直接通道,实质上取消了仪表的定期维护。

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